This invention relates to a readout apparatus for a ring laser angular rate sensor having improved resolution.
Ring laser angular rate sensors are well known in the art. An example of a ring laser angular rate sensor is illustrated in U.S. Pat. No. 3,373,650, issued to Killpatrick and assigned to the same assignee as the present invention. In such sensors, a portion of the counter-propagating laser beams are optically hetrodyne. Briefly, this may be accomplished by way of example using a prism which combines a portion of the counter-propagating laser beams at slightly different angles and thereby creates a fringe pattern across the surface of a detector array containing one or more photodetectors such as photosensitive diodes, each diode being smaller than an individual fringe spacing. A pair of photodetectors are typically separated by one quarter of a fringe spacing so as to provide output signals which are in phase quadrature. The photodetector output signals are utilized as a mechanism for counting the number of fringe spacings passing thereby. Each fringe passing the photodetector represents a precise amount of rotation angle. The rate of change of the fringe changes being indicative of the rotation rate. Two detectors are usually employed and spaced as aforesaid to provide a pair of signals which can be utilized to determine rotation direction. An example of a photodetector array, by way of example, is illustrated in U.S. Pat. No. 4,152,072 issued to Hutchings.
When the output of a single photodetector responsive to the interference pattern is used as the primary readout, the resolution is limited to one fringe change or count which can be interpreted as a precise measurement of rotation angle. The object of the present invention is to provide a readout having four times the resolution of a single photodetector output signal.